Many workpieces currently being manufactured require the drilling of extremely small diameter holes through the workpieces. Such holes are sometimes referred to as "through holes." An example of such is a turbine blade for a jet engine which requires the drilling of numerous cooling holes in various portions of the blades. Because of the small diameter of such holes, they are commonly drilled using a laser drilling process, which process is controlled by regulating the amount of energy per pulse and the number of pulses per hole. Typically, a machine control is programmed to effect the delivery of a predetermined number of shots or laser pulses for each specific hole. Due to variations inherent in the manufacture of the undrilled parts, the thickness of metal walls in such parts is not always constant. When a wall is too thick, an incomplete hole may be drilled (underdrilling) and redrilling of the part is required. If the wall is thinner than expected, the programmed number of laser pulses will penetrate the wall and may scar any material in the laser beam path (overdrilling). This overdrill condition results in added expense for material evaluation, reinspection, and sometimes parts scrapping.
It is preferable in laser beam drilling of workpieces with limited and sometimes inaccessible internal spaces, such as jet engine turbine blades, to determine if a hole is completely through by looking at the workpiece from the front or beam side with a radiation sensing system. However, in the operation of a pulse laser drill, the laser pulse beam striking the material to be drilled, results in the production of very high temperatures by intense local heating in as much as laser drilling is a thermal process. The intense local heating causes very hot minute particles of material to be expelled from the localized drilling area. The heated minute particles, in the form of a plume of sparks, radiate heat and light, i.e. extrinsic radiation. When a radiation sensing system looking for intrinsic laser pulse radiation is used to determine if a hole is completely through, the extrinsic radiation from the hot minute particles causes false readings in the radiation sensing system.
It is an object of the present invention to provide a method and apparatus for detecting when a through bore is drilled in a workpiece.
It is a further object of the present invention to provide a method and apparatus for discriminating between intrinsic radiation and extrinsic radiation in a pulse laser drilling operation.
It is still another object of the present invention to provide an apparatus and method which prevents further drilling once a through bore is drilled in the workpiece.
It is yet another object of the present invention to provide an apparatus and method which prevents underdrilling in a pulse laser drilling operation.